Genetic organization of the human MHC class III region

The human major histocompatibility complex (MHC), or human leukocyte antige n (HLA) region, encompasses over 4 Mb of DNA on the short arm of chromosome 6 and is traditionally divided into the class I, class II and class III re gions. The MHC has now been entirely sequenced and similar to 220 genes hav e been defined of which similar to 62 are in the class III region. It is be coming clear that many of the latter encode proteins that are likely to be involved in the immune and inflammatory responses. The MHC is known to cont ribute to a large number of immune-related disorders including insulin depe ndent diabetes mellitus, rheumatoid arthritis, common variable immunodefici ency and IgA deficiency and there is growing evidence that genes within the class III region are important in determining susceptibility to many of th ese complex conditions. Genes in the class III region have also been implic ated in a number of non-immune-related diseases such as congenital adrenal hyperplasia and sialidosis. Now that the full gene content of the class III region is known the stage is set for the identification and characterisati on of candidate disease genes, which will allow greater understanding of th e causes of many MHC-linked diseases and thus aid the development of improv ed treatments.